There is an engine which is a bit underpowered when idling, it can not even keep the car moving on a flat road when in gear and idling at 700..800 RPM - see link.

What can be done to improve its torque at low RPMs?

I am looking for a list of things to check, each of which can help a tiny bit, and maybe together they help enough. For example things like cleaning/changing the air filter? What maintenance/repair can be done to the engine itself (besides changing oil) that might improve the torque at low RPM?

Not actually a solution, more of a workaround: can you raise the idle speed slightly? I know it is supposed to run happily at 700-800 but I had use of a very old truck that had a problem with torque at idle that we failed to diagnose (it wasn't worth much so we didn't try too hard) and our fix was to make it idle about 120rpm faster. Solved the issue.